If the front driveshaft locks in, the transfer case, its controls, and the TCCM are O.K. Make sure the actuator under the battery tray is getting full vacuum and can move the lock pin in the front differential.
You should be able to manually compress the vacuum actuator and move the cable, and thus the differential lock pin. If the cable is damaged the system won't work. If vacuum is inadequate, the system won't work.